A novel obligately anaerobic, mesophilic, organotrophic bacterium, strain P3M-1(T), was isolated from a microbial mat formed in a wooden bath filled with hot water emerging from a 2775 m-deep well in the Tomsk region of western Siberia, Russia. Cells of strain P3M-1(T) were rod-shaped, 0.3-0.7 µm in width and formed multicellullar filaments that reached up to 400 µm in length. Strain P3M-1(T) grew optimally at 42-45 °C, pH 7.5-8.0, and with 0.1% (w/v) NaCl. Under optimal conditions, the doubling time was 6 h. The isolate was able to ferment a variety of proteinaceous substrates and sugars, including microcrystalline cellulose. Acetate, ethanol and H(2) were the main products of glucose fermentation. The genomic DNA G+C content was 55 mol%. 16S rRNA gene sequence-based phylogenetic analyses showed that strain P3M-1(T) was a member of the class Anaerolinea, with 92.8 % sequence similarity to Levilinea saccharolytica KIBI-1(T). Based on phylogenetic analysis and physiological properties, strain P3M-1(T) represents a novel species in a new genus, for which the name Ornatilinea apprima gen. nov., sp. nov. is proposed; the type strain of O. apprima is P3M-1(T) (= DSM 23815(T)=VKM B-2669(T)).